Obesity is increasing significantly in affluent societies, this trend includes children and adolescents. Weight gain results from caloric intake exceeding caloric (energy) expenditure. The increase in prevalence of obesity represents a population shift towards positive energy balance. Dietary intake and physical activity represent the behavioral, and therefore modifiable aspects of this balance equation, unlike energy expenditure. It has been shown that reduced energy expenditure may constitute a risk factor for obesity for infants of overweight mothers, Pima Indian adults, and children with Prader-Willi syndrome, although this is less clear in other populations. Recent studies show that physical exercise has a pronounced protective effect against weight gain in adults. These studies have not been performed in otherwise healthy obese children and adolescents. This study will assess the effect of a low-fat (<30% total calories) diet and a moderate intensity exercise program in healthy obese children and adolescents upon energy expenditure and body composition compared to control patients: obese children who receive "standard" advice (without formal follow-up) for weight management. These determinations will be accomplished using indirect calorimetry, doubly labeled water (DLW), and DEXA (dual energy x-ray absorptiometry) analysis. The long-term goals of this project are to improve the understanding and recommendations for the prevention and treatment of obesity in children.